1. Field of the Invention
The invention relates generally to loudspeaker enclosures utilized for sound reproduction and particularly to a method and apparatus for more fully utilizing existing driver cone radiated energy for improvement of efficiency and quality of sound.
2. Description of the Prior Art
Conventional sound reproduction centers around the use of audio energy electrical/mechanical converters technically referred to as speakers or drivers. These drivers are composed of a cone shaped material with a coil of wire wrapped circularly around the smaller end of the cone. This wire, known as the voice coil, is immersed in a magnetic field and is driven by electric signals which, when sent through the wire, causes a reaction similar in motion to a piston to force the cone forwardly and rewardly and to create pressure and rarefaction waves within the surrounding air, which waves radiate outwardly from both the front and rear of the cone in the form of audio or acoustic energy.
Conventional drivers are mounted in loudspeaker enclosures with the face of the enclosure being utilized as the radiator while the remainder of the enclosure is used as a sound or acoustic energy absorption device. In structures of this nature the driver is physically attached to the face plate and the enclosure has walls formed of nonresonant mateiral with a high sound absorption coefficient, the walls of such enclosures being of a relatively high mass and thickness in order to facilitate maximum sound absorption. In addition, these enclosures are usually filled or stuffed with sound absorbent material such as cotton, fiberglass, etc. Such conventional speaker structure intends the radiation of the principal sound from the front of the enclosure and provides for the reduction or control of sounds which emanate from the rear of the driver cone since sounds emanating from the back side of the cone are essentially 180.degree. out of phase with the forward sound and would effectively cancel the forward sound wave if the two were permitted to co-mingle. This 180.degree. out of phase sound pressure wave is normally referred to as the back wave and, in addition to possessing high orders of audio energy that must be controlled, reacts within the interior of the loudspeaker enclosure (which in reality is a chamber or series of chambers) to create standing waves of high energy sound plus a counterforce of nodes or low energy areas. In addition, any structural material in the vicinity is invaded through the molecular framework of the material by both the primary frequencies of the front and back waves plus all of the supporting harmonics thereof, the totality of which creates vibration resonances commensurate with the mass, tension and composition of the material utilized in the enclosure structure. A profusion of resonances is thus activated by the driver from the driver chamber or chambers, sides, top, bottom, back, etc., it being necessary to bring all of these resonances under some semblance of control if the audio reproduction is to be properly presented.
Control of enclosure oriented sound energy has been directly related to the ability to engage and rapidly convert these waves of pressure energy to other forms of energy. The frequency range of audio sound is such that the most practicable means, and hence the basic control method that has previously emerged, is the conversion of kinetic pressure energy into heat energy. This conversion process involves insertion of materials with very high fiber count into the pathway of the audio wave. In attempting to penetrate the material, the audio wave will cause the individual fibers of the material to vibrate, thus absorbing and converting the audio energy into heat energy. Materials possessing a very high fiber count, such as cotton, fiberglass, particle baord and the like are commonly used. Unfortunately, the efficiency of high fiber count material is quite low and no material has yet surfaced which can effectively absorb and dissipate audio frequencies of the size typically used for loudspeakers in sound reproduction systems. Within the state of the art, high degrees of sound absorption can only be realized by developing anechoic conditions. However, the attainment of anechoic conditions requires the use of expensive materials, specialized construction techniques and air volumes of excessively large proportions, all of which tend to make the anechoic application impractical for typical loudspeaker enclosures.
Accordingly, prior practices in the art have only been able to contain the diverse resonances and undesirable sounds within and emanating from loudspeaker enclosures to that level of efficiency and effectiveness constrained by the commonly available high fiber count materials. These materials have of necessity been used regardless of unfavorable mass and weight considerations and even with the recognition that the mateirals cannot differentiate between desirable and undesirable audio sounds. In spite of the shortcomings attendant to the prior practices thus enumerated, two predominant designs of loudspeaker enclosures have previously emerged and are almost exclusively constitute conventional practice, these designs being describable as the sealed enclosure, better known as the "infinite baffle," and the ported box enclosure, most commonly referred to as the "bass reflex."
In the infinite baffle design, the backwave is sealed within the enclosure. The concept involes the use of all solid walls, thereby resulting in the rear wave being prevented from engaging the front wave. Further, high fiber construction material is used to stuff the interior of the enclosure, the high fiber count suppressing the many resonances and unwanted enclosure sounds. In practice, the practical size of a sealed enclosure is severly limited in comparison to the length of the soundwaves encountered. Additionally, such structures suffer from the fact that within a sealed enclousre the front and back waves are separated by merely a very thin piece of material covering the driver cone. These aspects, when coupled with the problems associated with acoustic suppressing material, point to the conclusion that both loss of efficiency and quality of sound are inherent in the use of the infinite baffle design. Difficulties also exist with the bass reflex design which utilizes an open port or hole on the driver side of the enclosure, the size of the port being directly controlled by the volume within the enclosure, internal resonance of the enclosure, the effective area and efficiency of the cone of the driver and the resonance of the driver itself. Although somewhat controversial in that these design factors can be appropriately integrated, the basis for the bass reflex is that each driver operates separately within a dedicated chamber with independent volumes and a dedicated port. Properly designed, a bass reflex enclosure produces a rear wave which emanates from the port and which will be sufficiently delayed so that on emergence of the wave from the port the wave will appear to be in phase with that wave emanating from the front of the driver cone. Front to back wave interaction is thus decreased and the integration of both driver and chamber volume is accomplished. However, this design provides only a partial solution as constraints exist such as the fact that port effectiveness is realized only for those frequencies associated with driver and enclosure resonance. The remaining frequencies, representing some 95% of the audible range, are apparently somehow handled by the enclosure and the stuffing within the enclosure. Attempted resolution of the efficiency and the degradation of sound quality resulting from resonances, standing waves, nodes, etc., within such an enclosure has previously been addressed and relate to the same basic problems confronting the infinite baffle design. No known material exists which can adequately suppress acoustic energy using the construction techniques that are currently employed and still remain within plausible size and weight restraints inherent with loudspeaker enclosures.
While additional attempts to address the problems noted herein have included other approaches such as the use of passive radiators to lower enclosure resonance and to overcome backwave phasing, such approaches require relatively large surface areas coupled with extremely low mass to be truly effective. Even in the very best of operating conditions efficiency of the passive radiator is very low, especially when quantified against the parent driver. The mass of the passive radiator is the major problem as it effectively loads the cone of the active driver causing the cone to react as if more mass has been added directly to the cone but not firmly attached. These designs change the original design characteristics of the driver and inevitably introduce distortion. Accordingly, the low efficiency compressed air activated cones which comprise passive radiators are limited in application.
Examples of prior art structures which have addressed the problems noted above but without full solution are disclosed in the patents now listed, these patents being exemplary of the prior art:
U.S. Pat. No. 2,166,838 Anderson PA0 U.S. Pat. No. 2,694,462 Robbins et al PA0 U.S. Pat. No. 2,840,181 Wildman PA0 U.S. Pat. No. 4,207,963 Klasco PA0 U.S. Pat. No. 4,284,844 Belles PA0 U.S. Pat. No. 4,301,332 Dusanek PA0 U.S. Pat. No. 4,398,619 Daniel PA0 U.S. Pat. No. 4,439,644 Bruney, III
The present invention provides solution to the problems noted above by provision of a simple mechanical device which either solely or in conjunction with programmed distribution of enclosure mass acoustically couples drivers such as conventional cone drivers both to the air and to the material from which the enclosure is formed.